begin low-level runtime implementation

llrtc/lib/udivmod64.c

@@ -0,0 +1,84 @@
+/*
+Implements unsigned divmod using for platform missing 64-bit division and/or
+modulo functions.
+*/
+#include "udivmod64.h"
+
+/*
+count left zero for 64-bit words
+
+*/
+static
+int clz64(uint64_t x)
+{
+    const int total_bits = sizeof(x) * BITS_PER_BYTE;
+    int zc = 0;
+
+    while (zc < total_bits && ((x >> (total_bits - zc - 1)) & 1) == 0) {
+        ++zc;
+    }
+    return zc;
+}
+
+typedef struct div_state_
+{
+    uint64_t tmp, dvd;
+} div_state;
+
+/*
+Left shift div_state by 1 bit
+*/
+static
+void div_state_lshift(div_state *state)
+{
+    state->tmp = (state->tmp << 1) | (state->dvd >> 63);
+    state->dvd = state->dvd << 1;
+}
+
+/*
+Division of unsigned 64-bit word using 64-bit addition and subtration following
+the shift-restore division algorithm.
+For those interested in 32-bit implementation,
+mapping of 64-bit addition and subtraction to 32-bit should be trivial.
+
+Reference: 
+    - IBM. The PowerPC Compiler Writer's Guide
+    - LLVM compiler-rt
+
+Assumptions:
+    - all operands and results are positive
+    - unsigned wrapped around
+*/
+uint64_t udivmod64(uint64_t dividend, uint64_t divisor, uint64_t *remainder)
+{
+    div_state state = {0, dividend};
+    uint64_t quotient = 0;
+    int i;
+    int skipahead;
+
+    if (divisor == 0) {
+        return 1 / 0;               /* intentionally div by zero */
+    }
+
+    /*
+    skipahead to reduce iteration
+    */
+    skipahead = clz64(dividend);
+
+    for (i = 0; i < skipahead; ++i) {
+        div_state_lshift(&state);
+    }
+
+    /*
+    division loop
+    */
+    for (i = skipahead; i < 64; ++i) {
+        div_state_lshift(&state);
+        if (state.tmp >= divisor) {
+            state.tmp = state.tmp - divisor;
+            quotient |= 1ull << (63 - i);
+        }
+    }
+    *remainder = state.tmp;
+    return quotient;
+}